1. Field of the Invention
The invention relates to the field of cooling arrangements for nuclear reactors when in a shutdown mode. In particular, the invention provides a cooling system that is separately powered and operable independently of the residual heat removal system of the reactor, and is arranged to cool the seals of the primary reactor coolant pump(s) and the primary core coolant.
2. Prior Art
Pressurized water and boiling water nuclear reactors having a number of cooling systems operable during different phases of reactor operation to remove heat produced by nuclear fission in the reactor core. The primary operational function of the reactor is to heat a liquid coolant that is pumped through a primary coolant circuit having the reactor vessel in series with means for converting the heat energy in the coolant to motive energy, for example to operate an electrical generator. This primary operational function can be considered a cooling function, i.e., cooling the reactor core, as well as an energy transfer function.
In a pressurized water reactor, the primary coolant circuit includes a stream generator in series with the reactor, for producing stream in coolant water that is isolated from the primary coolant by a heat exchanger. The steam produced by the steam generator drives a turbine coupled to an electric generator. Thus the primary coolant circuit removes heat energy from the reactor core and moves it to the stream generator.
In a pressurized water reactor, the primary coolant circuit is operated at substantial pressure (e.g., 150 bar) such that the water does not boil at the substantial temperature to which the coolant is heated (e.g., 30.degree. C.). One or more reactor coolant pumps circulates the coolant in the loop including the respective heating and heating-dissipating (energy extracting) elements. This coolant pump requires shaft sealing to maintain the pressure barrier, and the coolant pump seals can be cooled by a further flow of coolant (normally from a different source than the primary coolant circuit), to maintain the integrity of the seals.
A second cooling funtion is provided for safety reasons, to deal with the possibility of a loss of primary coolant circuit function during operation of the reactor. A breach in the primary coolant circuit, for example, could allow the core to overheat, resulting in damage to the nuclear fuel. A pressurizer arrangement injects additional coolant into the circuit to maintain operational pressure and to replace coolant that may be lost through a minor breach or leak in the coolant circuit. Various techniques are known for cooling the reactor core in the event of a major breach such as the rupture of a conduit in the primary coolant circuit. Neutron absorbing control rods can be inserted into the fuel array quickly to damp the nuclear reaction, for example when the sensed coolant pressure drops. However, it remains necessary to cool the operationally-heated fuel. A volume of emergency cooling water can be maintained, to be pumped or released by gravity into reactor vessel, such that the emergency cooling water can cool the core. Such an arrangement can involve circulating the emergency coolant, such as by condening and recycling stream released from the coolant water when boiledby the hot core. Alternatively or in additiion, one or more heat exchangers can be used to move heat from the coolant to some external sink.
A third cooling function applies when the reactor is not operational but the nuclear fuel in the reactor vessel continues to generate heat due to nuclear decay. Residual heat removal arrangements, such as disclosed in U.S. Pat. No. 4,113,561-Fidler et al, provide additional conduits, pumps and heat exchangers for removing heat from the core when the reactor is not operating to generate electric power. Such systems may be coupled directly to the primary coolant circuit as in Fidler et al, or coupled through heat exchangers as in U.S. Pat. No. 4,830,815-Gluntz.
Arrangements for emergency cooling and those for residual heat removal are similar to one another and similar to operational power generation in that each is directed to moving heat energy away from the core. However, the source of the cooling water employed, the manner in which the particular cooling system is powered, the pressure at which the system must operate, the cooling capacity required in view of precisely how the reactor is cooled, the relative gravity of the situation, and other aspects are quite different.
Most nuclear power plants have several sources of electric power, including the power generated locally the turbine/generator, offsite power from the normal electric power grid, and emergency power generated by emergency diesel generators. Typically, two emergency diesel generators are provided such that one generator is available if the other should fail to operate. The emergency generators are "safety grade," and in design planning to prepare for potential accidents and similar contingencies, at least one of the emergency generators typically is assumed to be available for powering shutdown functions and emergency cooling in the event of a design basis accident during operation of the reactor. Similarly, at least one diesel generator is assumed to be available for powering residual heat removal functions when the reactor is not generating operational power, that is, during shutdown. As with many safety systems employed with nuclear reactors, the emergency generators are designed for high reliability and automatic actuation. The generators are physically separated from one another to reduce the likelihood that both will be damaged by a forseeable, if unlikely, accident. Nevertheless, the assumption that at least one emergency power source will always be available is questionable. During shutdown, power generated locally by the reactor is not available. It is not inconceivable that in the event of a major disruption, power from the power grid and both of the two emergency generators may be unavailable as well. (An actual occurrence of this situation is described in US NRC Document NUREG-1410.)
If none of the respective power source is available, there are two primary concerns applicable durnig shutdown. If the reactor coolant system is hot and pressurized, a first concern is cooling the seals of the reactor coolant pumps to maintain the pressure barrier. With loss of cooling, the hot reactor coolant seals degrade from the effects of hot coolant, causing increased leakage of coolant and unintended depressurization. When the reactor core is in shutdown and depressurized, a further concern is removal of decay heat that is still being generated by the core. If all poer is lost and cooling functoins are disabled, the reactor coolant system will reheat, and could boil away the coolant, leaving the reactor core without any means to remove heat generated by the core.
Whereas existing nuclear power plant designs provide pump seal cooling and residual heat removal functions using the primary valves, pumps, heat exchangers and other service elements that operate when power is available, it would be advantageous to provide a cooling system that is not dependent on the design basis emergency backup diesel generators, and is useful during shutdown to maintain minimal cooling functions, including at least cooling of the reactor coolant pump seals, even when all other sources of power are lost.